1. Field of the Invention
The present invention relates to a driver circuit that applies a drive voltage to a driven load, and in particular to an ink jet recording head driver circuit that applies a drive voltage to an ink jet recording head including, e.g., one or more piezoelectric elements.
2. Discussion of Related Art
In recent years, impact printers have been replaced with non-impact printers and the non-impact printers have expanded their market. Out of various sorts of non-impact printers, an ink jet printer can operate on the basis of one of the simplest principles and can easily print a multi-gray-scale-step image and/or a full-color image. Out of various types of ink jet printers, a drop-on-demand ink jet printer that ejects only droplets of ink to be used to print images has been quickly popularized because of its high ejection efficiency and/or its low running cost.
Japanese Patent Application Publication P2000-280463A, Japanese Patent Application Publication P2000-211126A, or Japanese Patent No. 2689415 discloses an ink jet printer including an ink jet recording head having a piezoelectric element and an ink ejection nozzle, and additionally including a driver circuit that applies a pulse-like electric potential to the recording head so as to deform the piezoelectric element and thereby eject a droplet of ink from the nozzle. In the conventional ink jet recording head driver circuit, a so-called totem pole circuit, as shown in FIG. 6, is employed to apply the electric potential to the piezoelectric element. In the totem pole circuit of FIG. 6, a first bipolar transistor Tra and a second bipolar transistor Trb are connected in series to a positive-potential electric voltage source Vcc and a ground GND, and a piezoelectric element is connected via a resistor R to a connection point where the two transistors Tra, Trb are connected to each other. A control signal Ina is applied to a base of the first transistor Tra, and an inverted control signal Inb, produced by inverting the control signal Ina using an inverter INV, is applied to a base of the second transistor Trb. Since the second transistor Trb is turned on when the first transistor Tra is turned off, an electric charge of the piezoelectric element as a sort of capacitative load is discharged via the second transistor Trb to the ground GND. Thus, the pulse-like electric potential applied to the piezoelectric element can fall steeply. To control accurately the time when the pulse-like electric potential falls is essentially needed to operate the piezoelectric element to eject efficiently the droplets of ink. To this end, the ink jet recording head driver circuit employs the totem pole circuit.
However, it has been found that in the above indicated totem pole circuit, when the first transistor Tra is changed from its OFF (turned-off) state to its ON (turned-on) state, the changing of the second transistor Trb from its ON state to its OFF state may be delayed. In this event, the first and second transistors Tra, Trb are simultaneously placed in their ON states for a moment. The reason why the two transistors Tra, Trb are simultaneously placed in their ON states will be explained below by reference to a time chart shown in, FIG. 7. FIG. 7 shows a waveform of the control signal Ina, a waveform of the inverted control signal Inb, an operation state of the first transistor Tra, and an operation state of the second transistor Trb.
When the control signal Ina is changed from its low level to its high level, an electric voltage of the control signal Ina gradually increases between a time t11 and a time t13. Here it is noted that this transitional change of the signal voltage is, in fact, represented by a complex quadratic curve but, in FIG. 7, it is represented by a simple straight line. In the conventional driver circuit, a threshold voltage value of the inverter INV is pre-set at 50% of the highest electric potential of the control signal Ina, i.e., an average of the low level and the high level of the signal Ina. Therefore, when the control signal Ina is changed from its low level to its high level, an output of the inverter INV, i.e., the inverted control signal Ina starts changing from its high level to its low level, at a time t12 when the electric potential of the control signal Ina becomes equal to the above-indicated threshold voltage, 50%, of the inverter INV. In addition, a threshold voltage value of the first transistor Tra is pre-set at 50% of the highest electric potential of the control signal Ina. Therefore, at time t12 when the electric potential of the control signal Ina becomes equal to the threshold value, 50%, of the first transistor Tra, the first transistor Tra is turned on. On the other hand, a threshold voltage value of the second transistor Trb is pre-set at 50% of the highest electric potential of the inverted control signal Inb. Therefore, at a time t13 when the electric potential of the inverted control signal Inb becomes equal to the threshold value, 50%, of the second transistor Trb, the second transistor Trb is turned off. Thus, the first and second transistors Tra, Trb are simultaneously placed in their ON states for a time duration between time t12 and time t13.
If the first and second transistors Tra, Trb are simultaneously placed in their ON states, then a high electric potential (e.g., 20 V) of the electric voltage source Vcc that drives the piezoelectric element is applied to the ground GND, so that an electric potential of the ground GND is instantaneously increased, which may lead to causing a malfunction of a driver IC (integrated circuit) as part of the driver circuit. In addition, the electric potential of the ground GND may exceed a drive voltage (e.g., 3.3 V) of other transistors of the driver IC, so that a back bias may be applied to the other transistors and thereby cause disorders of the same.